A general trend is the miniaturization of components. In terms of capacitors, new technologies have been developed to integrate thin-film capacitors with high capacitance densities on silicon chips. Two main technology routes are followed: trench capacitors or complex oxide capacitors such as ferroelectric capacitors. Both capacitor types are suited for radio frequency (RF) decoupling. But the capacitors are still not small enough to integrate μF capacitor values on a reasonably small area.
Conventional integrated capacitors store the electrical energy as charge on two parallel plates placed sufficiently close together. Using the parallel plate capacitor concept there are three ways to increase the capacitance density: i.e. reduce the spacing between the parallel plates, increase the dielectric constant of the dielectric material, or increase the surface area (i.e. fold the 2-dimensional capacitor into a 3D space). Bearing in mind present-day requirements for leakage current, breakdown voltage, and reliability, the state of the art does not allow to further decrease the dielectric thickness, or further increase the dielectric constant. Moreover, the dielectric deposition in 3D structures is not trivial and adds significantly to the manufacturing costs of these devices.
Some applications, e.g., supply line decoupling or pulsed operation modes, require large capacitor values. The large values cannot be integrated easily. For instance, a three dimensional MIM capacitor with a 20× surface enlargement can reach a maximum capacitance density of 200 nF/mm2 with an erbium doped HfO2 high-k material and a dielectric thickness of 10 nm. Note that “three dimensional” refers to a capacitor with a surface structured, e.g., using a trench structure, to have a capacitor electrode area greater than the area that the capacitor covers on the substrate surface. Thinner films cannot be applied to avoid excessive leakage currents and early breakdown. For a voltage range of 2.0 V the total amount of usable charge of such a capacitor is 0.4 μC/mm2 or 0.11 nWh/mm2.
A possible technology to manufacture ultra-high density capacitors (>1 μF/mm2) is electrochemical capacitors. Electrochemical capacitors store the energy by moving ions in electric double layers or store electrical energy as chemical energy by means of redox reactions.
Typically, such electrochemical capacitors use electrodes of activated carbon and an electrolyte. A two dimensional (flat electrodes) electrochemical capacitor with a cathode thickness of 100 nm has a capacity of 45 nWh/mm2 or 0.6 mC/mm2, i.e. a more than 1000× larger energy density than conventional capacitors.
Although the energy storage can be increased by more than 3 orders of magnitude using such an electrochemical capacitor compared to more conventional capacitors, since the volume determines the storage capacity not the surface area, the release of the energy is limited due to the fact that the internal resistance is high i.e. ions should move from one electrode to the other.
Further, electrochemical capacitors use liquid or gelled electrolytes and electrode materials with relatively slow ion conduction. Therefore, the capacitance will drop significantly at high frequencies. The decoupling potential decreases and the maximum charging or discharging current is limited by the ion conductivity. Moreover the stability at high temperatures is limited due to the usage of liquid/polymeric electrolytes which blocks integration on chip. Therefore, standard electrochemical capacitors cannot replace the existing high-k capacitors, resulting in extra area and cost, when they are combined.
An alternative to electrochemical capacitors for charge storage in some applications is battery technology, for example lithium ion battery technology. In such technology, lithium ions are used as the means for electric transport through an electrolyte. Typically, the anode and cathode of a lithium ion battery use lithium ion intercalating technology, as illustrated schematically in FIG. 1. In this arrangement, lithium ions are intercalated into the matrix of the anode and the cathode.
FIG. 1 shows a cathode of an electrochemical capacitor showing two states, one (the left) having no lithium ions embedded and one (the right) having lithium ions embedded. The matrix is a metal oxide with metal atoms 2 and oxygen atoms 4 and gaps 6 which can accept lithium ions. These lithium ions 8 are shown intercalated in the right hand of FIG. 1. FIG. 1 is a schematic drawing with extreme states. Typically the lithium ions are not fully removed, but a fraction stays in the lattice.
However, lithium ion batteries are not capacitors and cannot be used in all applications. In particular, lithium ion batteries have discrete voltage outputs—unlike a capacitor, the voltage across a battery is not a smooth function of the charge stored. Instead, if the voltage across the battery is plotted as a function of charge, the plot has well defined plateaus, for example a plateau at 3.4 V. A comparative example is presented below (FIG. 4) which shows such a plateau. Of course, for a battery application such a plateau gives a relatively constant output voltage which is desirable in a battery, but such a plateau is not suitable for a capacitor.
Accordingly, there remains a need for a design of electrochemical capacitor that can be manufactured on a silicon substrate and not as a large discrete component, together with a corresponding manufacturing method.